Research. Expression of the HIV-I receptor CD4 on many non-human and see human cell lines is not sufficient to permit HIV-I infection. The investigator describes human glioblastoma cell line (U373) which remains resistant to HIV-1 despite the added expression of an authentic CD4 molecule. The block to H1V- 1 infection of these cells is strain- independent and appears to be at viral entry. Heterokaryons of CD4- expressing U373 (U373-CD4) cells fused to HeLa cells allow HIV-I entry. A U373-CD4\HeLa hybrid clone allows efficient HIV-I replication. These results suggest that HeLa cells express a factor(s) that can complement the viral entry defect of U373-CD4 cells and which is necessary for efficient HIV-l infection. Experiments that test the ability of additional human cell lines to complement U373-CD4 cells will provide insight into the complexity of the viral entry process. Studies to identify the chromosome encoding the complementing gene and the gene itself will help define the cellular requirements for CD4-mediated HIV-l infection and may provide new targets for interrupting the viral life cycle.